Occurrence of Shortleaf Loblolly Pine Hybrids in Shortleaf Pine Orchards: Implications for Ecosystem Restoration

Shortleaf pine (Pinus echinata Mill.) is an important conifer in much of the southeastern United States. However, the species and its associated ecosystems are in decline, and recent evidence about hybridization with loblolly pine (Pinus taeda L.) raises concerns that the species may be at risk of further losses due to introgression. Although shortleaf pine is not widely planted for timber production, the US Department of Agriculture (USDA) Forest Service, state forestry agencies, some conservation organizations, and private landowners use artificial regeneration to regenerate shortleaf pine for various purposes including restoration of shortleaf pine-dominated ecosystems. Given the threat of introgression with loblolly pine, we tested federal and state shortleaf pine seed orchard trees (i.e., grafted clones) and wind-pollinated seedlings (from various nurseries sourced from several of the tested orchards) for hybrid character, using a previously developed microsatellite DNA test. We found that 8 to 10% of the USDA Forest Service orchard clones and 0 to 10% of state agency clones genetically resembled F1 hybrids or first-generation backcrosses to shortleaf pine (SLBC1). Frequencies of hybrid classifications in seedlings were generally similar to those of their seed orchard parent trees (0 –10%), although seedlings from the Oklahoma Forestry Services nursery contained an unusually high proportion of apparent F1 hybrids (4%), possibly due to the proximity of a loblolly pine seed orchard and loblolly pine plantations to the shortleaf pine seed orchard of origin. Based on these results, we recommend that shortleaf pine seed orchard managers consider steps to mitigate the genetic impact of trees classified as either F1 or SLBC1 hybrids to maintain the genetic integrity and desired phenotypic traits (i.e., fire, drought, and ice tolerance) of their shortleaf pine seeds, ensuring survival and adaptation of the species and its ecosystem to future climate variation.